Photosensitive layers are useful in photosensitive reproduction elements in which differences in tackiness between the exposed and unexposed areas of the photosensitive layer are used to produce the image.
Image reproduction processes are known from German Pat. Nos. 1,904,058, 1,904,059, 1,210,321, and 2,004,214, in which tacky, photopolymerizable reproduction elements are hardened by imagewise exposure, i.e., the exposed areas of the image lose their tackiness. The photopolymerizable reproduction elements comprise a support film and a photopolymerizable layer, which contains at least one addition polymerizable monomer and a photopolymerization initiator.
Combinations of hexaarylbisimidazoles and mercapto compounds are described as initiator systems for the initiation of photopolymerization in, for example, German Pat. Nos. 1,904,058 and 2,004,214, as well as in U.S. Pat. No. 3,479,185. The hexaarylbisimidazole upon exposure is split into two free radicals, which react with the mercapto compounds so that new radicals are formed, which in turn initiate addition polymerization.
The effectiveness of the initiator system in terms of the sensitivity of the photosensitive layer is substantially increased by the mercapto compounds. The concentration of hexaarylbismidazole compound in such photopolymerizable reproduction elements generally amounts to 1.5 to 4.5% by weight, the concentration of the mercaptocoinitiator amounts to 0.2 to 2.5% by weight based on the total solid content of the photosensitive layer. An increase in this concentration, as stated in U.S. Pat. No. 3,479,185, does not lead to a further increase of sensitivity, but in some instances even leads to a regression of sensitivity.
The latent image areas of varied tackiness formed after exposure are made visible by the application of suitable colored toners, which only adhere to the unexposed tacky areas. The toners are easily removed from the exposed nontacky areas. Positive images of the original are obtained according to these known processes which have attained considerable importance for carrying out color proof processes in reproduction technology.
An element is known from Abele and Grossa U.S. Pat. No. 4,243,741, which is not based on photopolymerization and in which the unexposed areas are nontacky. The exposed areas upon imagewise exposure become tacky and can be made visible using a suitable toner thereby forming a negative image. This element utilizes a dihydropyridine compound and a hexaarylbisimidazole compound.
The inherent yellow color of the dihydropyridines is disadvantageous with this element and limits the use of the element for color proofing purposes. The inherent yellow color of the layers is enhanced proportionately to the number of layers employed in the preparation of multilayer, multicolor images, so that the background (nontoned) areas do not exhibit the desired freedom from unwanted yellow coloration.
An object of this invention is to provide a photosensitive reproduction element for the preparation of negative tonable images which avoids the disadvantage of yellowing.